How to run a cron job inside a docker container?

Question

I am trying to run a cronjob inside a docker container that invokes a shell script.

Yesterday I have been searching all over the web and stack overflow, but I could

Answer 1

I created a Docker image based on the other answers, which can be used like

docker run -v "/path/to/cron:/etc/cron.d/crontab" gaafar/cron

where /path/to/cron: absolute path to crontab file, or you can use it as a base in a Dockerfile:

FROM gaafar/cron

# COPY crontab file in the cron directory
COPY crontab /etc/cron.d/crontab

# Add your commands here

For reference, the image is here.

Answer 2

Here's my docker-compose based solution:

  cron:
    image: alpine:3.10
    command: crond -f -d 8
    depends_on:
      - servicename
    volumes:
      - './conf/cron:/etc/crontabs/root:z'
    restart: unless-stopped

the lines with cron entries are on the ./conf/cron file.

Note: this won't run commands that aren't on the alpine image.